Update server side templates to use Jinja

Dokku: https://jinja.dokku.hypothes.is

Jinja[1] is a far more front-end friendly tempting system that is similar to the language that Django[2] uses, (it also has analogs in PHP[3] and Node[4]) so it's familiar to a lot of people.

This pull request...

* Swaps out chameleon for Jinja across the board.
* Moves angular templates, layouts and helpers into their own directories.
* Renames embed.txt to embed.js to avoid confusion around the content type.
* Includes a custom Jinja extension for including raw content (used for including Angular templates).

[1] http://jinja.pocoo.org/docs/dev/
[2] https://docs.djangoproject.com/en/dev/topics/templates/
[3] http://twig.sensiolabs.org
[4] https://mozilla.github.io/nunjucks/
Could you link to the part of web platform that overrides it? From this description it would seem simpler to me for us to just set template paths here and then reference them directly in app.html.

```
{% include auth_template %}
{% include account_template %}
```

Does that make sense?